Downhole fluid analysis may be performed to estimate the level of contamination in downhole fluid samples and to provide information on the composition and properties of formation fluids. One example of a technique used for downhole fluid analysis is optical spectroscopy. In optical spectroscopy, the fluid sample of interest is illuminated with photons (electromagnetic waves) and the fluid may absorb photons having one or more different energies (wavelengths). The particular energy absorbed is related to the chemical composition and physical nature of the atoms and molecules making up the fluid and thus, the optical absorption/transmission measured as a function of energy (wavelength) of the photon, referred to herein as an optical absorption/transmission spectrum, may be used to estimate the chemical composition of the fluid.
Optical spectra are traditionally recorded in transmission mode. In transmission mode, photons are directed through the fluid sample towards a photon detector, and the number of photons transmitted through the sample is recorded. Photons that do not pass through the sample are assumed to have been absorbed by the sample. However, a second process can occur that may also prevent photons from passing through the sample and reaching the detector. This second process is referred to herein as scattering. As opposed to absorption, scattering of photons may not necessarily provide the desired information regarding the chemical composition of the sample or may serve to greatly complicate the interpretation of any measured optical spectrum.
In some conventional oil and gas reservoirs, scattering may often be ignored. However, scattering in some oils, such as heavy oils, cannot be ignored and in fact typically dominates over absorption. As a result, the absorption spectrum of heavy oils cannot be measured by traditional optical spectroscopy in transmission mode.